1. Field of the Invention
The present invention relates to an improved method of micellar flooding and, more particularly, to such a method which utilizes optimum grading of the chemical concentrations in the injected fluid banks.
2. Setting of the Invention
After the oil production from a formation has declined below a certain production rate, some form of secondary and/or tertiary recovery method is often employed to recover the remaining inplace oil. One form of tertiary recovery method involves the injection of a micellar bank of fluid into the formation followed by the injection of a mobility control bank of fluid to displace the oil and water towards the production wells.
Most micellar fluids are miscible with both oil and water when initially injected in the formation. However, due to the relatively small size or quantity of the micellar bank, it degenerates and loses miscibility after moving a short distance within the formation. Once the micellar bank degenerates, the fluid becomes either miscible with oil but immiscible with water, immiscible with both oil and water, or it becomes miscible with water but immiscible with oil. When the micellar fluid bank becomes immiscible with both oil and water, then a significant portion of the oil will be trapped within the formation and nonrecoverable. Some operators prefer to have the bank miscible with oil and not with water because almost all of the inplace oil may be recovered. However, because the micellar bank is small compared to the following polymer bank, the micellar fluid would be immiscible with the polymer bank. In this case, the polymer bank traps the micellar fluid in the porous rock media. When all of the active solution in the micellar bank has been trapped, the micellar displacement of the oil is lost and no additional tertiary oil is mobilized. The trapping of the degenerated micellar bank by the polymer water can be eliminated if a water micellar miscible fluid is used. In this case, the active solution loss is much lower and the micellar bank propagation through the formation is much greater. However, this increased propagation is at the expense of the amount of oil displaced since this fluid is immiscible with oil, any fraction of the oil will remain in the porous rock media.